Natural teeth may be lost as a result of dental disease or trauma making it desirable for replacement with prosthetic devices. One type of prosthetic device is a dental implant or root member which is surgically positioned within the mandibular or maxillary alveolar bone. After healing a head member or abutment is mounted on the implant and then, in turn, a tooth-simulating prosthesis, or crown, is mounted on the abutment.
In preparing a site for a generally cylindrical implant a bore having a diameter generally the same as the diameter of the implant is drilled in the aveolar bone. The implant is positioned in the bore and, for an implant type positioned below the surface of the bone, is packed with autogenous graft material, such as bone particles harvested during the drilling procedure. The implant is provided with an abutment receiving bore which is closed with a temporary healing plug and the site is covered and allowed to heal for a suitable period of time, e.g., 3-6 months to allow for osseointegration with the bone and the implant forming essentially a unitary body. After the healing and bone growth process the temporary healing plug is surgically accessed and removed. A permanent abutment having a post portion receivable in the bore of the implant is then mounted on the implant.
In the early stages of the development of dental implants a typical failure mode comprised the migration of epithelium along the sides of the implant causing bone loss until eventually the implant would fall out or break due to increased leverage forces. The advent of various grooves and laterally extending fins which promote osseointegration has largely overcome this problem, however, typically there is still some epithelial migration and concomitant bone loss at the crestal end of the implant. This tendency is exacerbated if there is any motion between the implant and the abutment.
In one type of implant system the abutment is threadingly attached to the implant. Although the screw threads are manufactured with very tight tolerances to minimize any looseness there is inherently a certain amount of micro-motion between the screw portion and the threaded bore of the implant to enable the screw to be screwed into the implant. This micro-motion and bacterial leakage between the components cause bone cratering and soft tissue attachment migration leading to further deterioration.
Another dental implant system utilizes an abutment having a post formed with a locking or self-holding taper which is received in the bore of the implant which is formed with a matching locking taper. In this type of system once the post is engaged in the bore of the abutment there is no motion whatsoever between the two members. A system of this type is shown and described in co-assigned U.S. Pat. No. 4,738,623, the subject matter of which is incorporated herein by this reference. In this patent an implant is shown having multiple, outwardly extending fins formed on the lower portion thereof and has a narrowed upwardly and inwardly contoured shoulder formed above the fins. In this system epithelial migration and bone loss is found to be limited to the top portion of the shoulder unless the patient has some systemic problem, e.g., the patient is a diabetic or a smoker.
It is an object of the present invention to provide a dental implant system having means to limit epithelial migration which can be used with implant systems employing either threaded engagement members or locking taper engagement members.
Another object of the invention relates to implants intended for posterior placement or other areas where minimal bone depth is available. Normally, the relatively long length of an implant provides a favorable length to width ratio which enables the implant to withstand the high lateral loading to which the implant is subjected. However, at the posterior part of the oral cavity, as well as other parts of the body, the depth of the bone is frequently insufficient for a normal length implant. Further, in the posterior section the lateral forces are greatest while at the same time the length to width ratio is decreased thereby resulting in a greater propensity toward implant failure.